Preheat your oven to 350°F (175°C) and grease your 9x9 inch baking pan or line it with parchment paper for easy removal later.
In a large mixing bowl, whisk together the melted unsalted butter, large egg, light brown sugar, and vanilla extract until well combined and smooth.
Gradually add the all-purpose flour to the wet mixture. Stir gently with a spatula until just combined. Be careful not to overmix!
Fold in the coarsely chopped Oreo cookies and M&Ms until they are evenly distributed throughout the batter.
Pour the batter into the prepared baking pan and spread it evenly. Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out clean.
Once baked, remove the pan from the oven and let it cool in the pan for about 10 minutes. Then, transfer to a wire rack to cool completely before cutting into bars.
